The locking of intramedullary nails is known in the art. The introduction of locking screws or locking bolts (hereinafter referred to collectively as “locking screws”) into the transverse bores of a intramedullary nail is carried out either with the aid of an imaging process (X-ray control) or a relatively complicated aiming device. In both cases, a certain aiming inaccuracy is unavoidable because the tip of the screw cannot be aligned exactly coaxially with the central axis of the transverse bore, and instead deviates therefrom by a certain amount. In order to enable the locking screw to enter into and pass through the transverse bore despite this aiming error, the outside diameter of the screw is underdimensioned such that it will be less than the diameter of the transverse bore. If the aiming accuracy remains within the range of this underdimensioning, the locking screw can be guided, despite the aiming error, through the transverse bore without any problem. In any case, as a result of the underdimensioning, the locking screw has a certain play or clearance relative to the transverse bore.
This clearance defines the amount by which the main bone fragments, which are fixed by means of locking screws in the corresponding locking hole, can move relative to the nail, and, because of the rigidity of the nail, move relative to the other main bone fragments fastened with the same nail. While some play or clearance is essential to guarantee the usefulness of the locking for surgeons, in the case of some indications (e.g. in the case of metaphysial fragments) it is clinically undesirable.
Even nails with a solid cross-section, that may have an internal thread in the locking hole, are not without clearance. The internal thread merely prevents the axial displacement of the nail on the locking screw.
U.S. Pat. No. 6,296,645 to Hover et al. discloses a hollow, metallic intramedullary nail with diametrally opposed openings on the jacket of the transverse bore, described as windows, having one or two plastic inserts through which the locking screw can be introduced. A disadvantage of this known intramedullary nail is that the window-like plastic inserts can be easily pushed in, such that their desired function is lost. Even with a careful manipulation, the two plastic inserts can be pushed out from their “windows,” which also leads to a loss of function.